The present invention relates to an image forming apparatus, such as a copying machine and a printer, which employs an electrophotographic, electrostatic, or the like recording system.
Conventionally, an image forming apparatus of a transfer type, such as a copying machine, a printer, a facsimile, or the like, which employs a transfer type electrophotographic recording system comprises: a photosensitive member as an image bearing member, which is generally in the form of a rotatable drum; a charging apparatus (charging process) for uniformly charging the photosensitive member to a predetermined polarity and a potential level; an exposing apparatus (exposing process), as an information writing means, for forming an electrostatic latent image on the uniformly charged photosensitive member; a developing apparatus (developing process) for developing the electrostatic latent image formed on the photosensitive member, into a toner image, or a visual image, with the use of toner, that is, developer; a transferring apparatus (transferring process) for transferring the toner image from the surface of the photosensitive drum onto transfer medium such as paper; a cleaning apparatus (cleaning process) for cleaning the surface of the photosensitive member by removing the toner remaining, although only in a small amount, on the photosensitive drum after the transfer process; a fixing apparatus (fixing process) for fixing the toner image on the transfer medium; and the like. The photosensitive member is repeatedly subjected to electrophotographic processes (charging, exposing, developing, transferring, and cleaning processes) to form an image.
The toner which remains on the photosensitive member after the transfer process is removed from the surface of the photosensitive member by the cleaning apparatus. The removed toner is collected by the cleaning apparatus, and remains as waste toner within the cleaning apparatus. From the standpoint of environmental protection, efficient usage of natural resources, and the like, it is desirable that such waste toner is not generated.
Thus, an image forming apparatus has been invented in which the so-called waste toner, that is, the toner which is recovered into the cleaning apparatus after the transfer process, is returned to the developing apparatus to be used again.
There has also been invented a cleaner-less type image forming apparatus, in other words, an image forming apparatus which lacks the cleaning apparatus, and in which the toner remaining on the photosensitive member after the transfer process is removed into the developing apparatus during the developing process, to be used again.
More specifically, the toner remaining on the photosensitive member after the transfer process, or transfer residual toner, is left untouched on the photosensitive member, and subjected to the following image forming processes. Thus, the photosensitive member with the transfer residual toner is charged, and exposed, to form an electrostatic latent image. Then, the transfer residual toner, that is, the toner which is present on the photosensitive member across the areas (non-image portions) which are not to be developed with toner, is recovered into the developing apparatus by a fog removal bias (difference Vback in potential level between the DC voltage applied to the developing apparatus and the surface voltage on the surface of the photosensitive member) in the process in which the electrostatic latent image is developed. According to this method, the transfer residual toner is recovered into the developing apparatus and used to develop the electrostatic latent image during the following rotation of the image bearing member. Therefore, no toner will be wasted, reducing the amount of the bothersome work of maintenance. In addition, being cleaner-less is advantageous in reducing image formation size.
a) A cleaner-less image forming apparatus such as the one described above, that is, an image forming apparatus in which the toner particles remaining on the photosensitive member after the transfer process are removed and recovered into the developing apparatus by the fog removal bias as the latent image on the photosensitive member is developed by the developing apparatus, has its own problem. That is, if a contact type charging apparatus which charges the surface of the photosensitive member, by coming in contact with the surface of the photosensitive member, is employed as the charging apparatus for such a cleaner-less image forming apparatus, some of the transfer residual toner particles, in particular, those the polarity of which has reversed from their normal polarity, adhere to the contact type charging apparatus, contaminating it to an intolerable level, as the transfer residual toner particles pass through the charging station, that is, the contact nip between the photosensitive member and contact type charging apparatus. This results in unsatisfactory charging of the photosensitive member.
More specifically, toner as developer mixedly contains toner particles the polarity of which has reversed from their normal polarity, although only by a small amount. Further, some of the toner particles with the normal polarity are reversed in polarity, or reduced in potential level, by transfer bias, separation discharge, and the like.
In other words, the transfer residual toner is a mixture of toner particles with the normal polarity, toner particles with the reverse polarity, and toner particles low in potential level. Thus, the toner particles with the reverse polarity, and the toner particles low in potential level, are liable to adhere to the contact type charging apparatus as they pass through the charging station, that is, the contact nip between the photosensitive member and contact type charging apparatus.
b) In order to remove and recover the transfer residual toner particles on the photosensitive member by the developing apparatus as the latent image is developed by the developing apparatus, it is necessary that the polarity of the transfer residual toner particles on the photosensitive member, which will be carried to the development station through the charging station, is normal, and also that their potential is at a level not impedimental to the development of the electrostatic latent image on the photosensitive member. The transfer residual toner particles with the reverse polarity, and the toner particles with an improper potential level, fail to be removed from the photosensitive member, and also fail to be recovered, by the developing apparatus, effecting an unsatisfactory image.
c) The adhesion of the transfer residual toner particles to the contact type charging apparatus, which was described in paragraph a), can be prevented by charging to a high potential level of the normal polarity, the transfer residual toner which is the mixture of the toner particles with the normal polarity, toner particles with the reverse polarity, and the toner particles with an insufficient potential level, so that the polarities of the transfer residual toner particles become uniformly normal.
However, after being charged by a means for controlling the potential level of the transfer residual toner particles to prevent their adhesion to the contact type charging apparatus, the potential level of the transfer residual toner particles is higher than a potential level not impedimental to the development of the electrostatic latent image on the photosensitive drum, making it difficult for them to be removed and recovered by the developing apparatus in the process in which the electrostatic latent image is developed by the developing apparatus. In this situation, the toner particles remaining on the photosensitive member overlap with the image portion formed during the following image forming rotation of the photosensitive drum, and as a result, an unsatisfactory image is produced.
With the diversification of user needs occurring in recent years, it has become necessary to continuously print images with a high printing ratio, for example, photographic images, as well as to produce color images which require multiple images to be developed in layers on the photosensitive member. Since the amount of transfer residual toner which these processes generate per rotation of the photosensitive member is much greater than that generated prior to the aforementioned diversification, the above described problem is exacerbated.
The primary object of the present invention is to provide an image forming apparatus in which the transfer residual toner particles do not adhere to the charging member.
Another object of the present invention is to provide an image forming apparatus in which the transfer residual toner particles can be recovered by the developing means.
According to an aspect of the present invention, there is provided an image forming apparatus that includes an image bearing member for bearing an electrostatic image; developing means for developing the electrostatic image on the image bearing member with toner electrically charged to a predetermined polarity; transfer means for transferring the toner image from the image bearing member to a transfer material; toner charging means for electrically charging residual toner remaining on the image bearing member to the same polarity as the predetermined polarity; and an image bearing member charging means for contacting to the image bearing member carrying the toner charged by the toner charging means to electrically charge the image bearing member to the same polarity as the predetermined polarity, the image bearing member charging means being effective to reduce a charge amount of the toner.